Gas flow control system for an analytical instrument

ABSTRACT

Gaseous sample mixture flows through two restrictive capillaries in series with each other on its way to a chemiluminescent reaction chamber. A bypass pump has its inlet connected to the junction of the capillaries and its outlet communicating with the atmosphere. A pressure regulator connects the inlet of the bypass pump with the atmosphere. The pressure regulator responds to the pressure difference between the junction of the capillaries and the reaction chamber to control the flow of atmospheric air to the bypass pump inlet and thereby maintains a constant sample mixture flow rate into the reaction chamber despite variations in the pressure of the source of the sample mixture.

United States Patent Colvin Sept. 25, 1973 [54] GAS FLOW CONTROL SYSTEMFOR AN 3,543,784 12/1970 Smith 137/8 X ANALYTICAL INSTRUMENT 3,128,7834/1964 Cowles et a]. 137/117 [75] Inventor: Alex D. Colvin, Oak Park,Mich. Prima'y Examiner Robert G Nilson [73] Assignee: Ford MotorCompany, Dearborn, y Zerschling et Mich.

[22] Filed: Nov. 1, 1971 [57] ABSTRACT Gaseous sample mixture flowsthrough two restrictive PP .1 194,245 capillaries in series with eachother on its way to a chemiluminescent reaction chamber. A bypass pumphas 52 us. (:1. 137/117 23/254 R inlet C'Onnected theiunctio Capillariesand [51 1111. C1. c0511 7/01 its Outlet Communicating with theatmosphere- A Pres" 58 Field of Search 137/2, 3, 7, 8, 9, W regulatorconnects the inlet of the bypass Pump 37/13 114 1 l7 1 23/254 R with theatmosphere. The pressure regulator responds to the pressure differencebetween the junction of the [56] References Cited capillaries and thereaction chamber to control the flow of atmospheric air to the bypasspump inlet and thereby UNITED 'S'IjATES PATENTS maintains a constantsample mixture flow rate into the fl i i reaction chamber despitevariations in the pressure of au tam 1 .1 3,285,703 11/1966 Narita et31.. 23 254 R the Source of the sample mlxture' 3,464,434 9/1969 Nielson137/7 X 4 Claims, 3 Drawing Figures pan 5e Pf-ABOUT sup/ 4 r 22 20 k9, 4/6 CAP/11,42) /4 CAP/ZZAE) r -1 J; 1 L 4Q e lo ezaz/mn/vs 1' Z6 28flfV/Cf 0120/10.

a0 Pen-sat: J8 P56756056 36 6,4065 1 6,4065

I k E @406! 42 i b I 32 l N 1 9 41 k M Q fi m/P s K 0 4 1 JOUfCf OX/Gf/VJOUECE GAS FLOW CONTROL SYSTEM FOR AN ANALYTICAL INSTRUMENTBACKGROUNDOF. THE INVENTION This invention relates to the subject matterof U.S. Pat. application Colvin et al. Ser. No. 146,927 entitledChemiluminescent Instrument" and Warnick et al. Ser. No. 146,929,entitled chemiluminescent Process both filed on May 15, 1971.

These related applications describe a chemiluminescent detector capableof measuring widely varying I amounts of nitric oxide in gaseousmixtures with excellent accuracy, sensitivity, and reproducibility.Gaseous sample mixtures from the atmosphere, the combustion products ofautomotive engines and other power plants including aircraft engines andfurnaces and gases involved in various chemical processes can beanalyzed on a continuous basis with the detector.

The chemiluminescent reaction utilized by the detector usually iscarried out at a pressure below atmosphericfilo produce the desiredreaction pressure, an

exhaust pump is connected to the outlet of. a reaction chamber andgaseous sample mixture and gaseous reactant mixture are throttled inrespective capillaries prior to entering the reaction chamber.Appropriate selection of the exhaust pump and the capillaries producesthe desired subatmospheric pressure in the reaction chamber.

This system operates satisfactorily when the gaseous sample mixture isavailable at a substantially constant pressure. To accommodate gaseoussample mixtures of varying pressures, pressure regulating devices areincluded in the flow path of the sample mixture. Such pressureregulating devices maintain a constant pressure, upstream of the samplecapillary and'generally perform adequately in maintaining a constantsample flow. The regulators usually include some polymeric materials,however, and the gaseous sample mixtures usually contain unburnedhydrocarbons, carbon monoxide, and nitrogen oxides that react therewith.Such reactions not only shorten the useful life of the regulator butalso change the composition of the sample mixture eventually reachingthe reaction chamber.

SUMMARY OF THE INVENTION This invention provides a flow control systemfor an analytical instrument capable of measuring the amount of agaseous constituent in a gaseous sample mixture that avoids contactbetween variable flow controlling devices and the gaseous sample mixtureeventually taking part in the analytical reaction. The flow controlsystem maintains a constant sample mixture flow rate despite widevariations in the pressure of the sample mixture source. In the system,a first flow restricting device in series with a second flow restrictingdevice connects the sample mixture source to the reaction chamber. Anexhaust pump connected to the reaction chamber outlet draws samplemixture through the flow restricting devices and into the reactionchamber. A bypass mechanism is connected to the junction of the flowrestricting devices and includes a bypass pump that draws off a portionof the gaseous mixture from the junction. A regulating device located inthe bypass mechanism responds to the pressure difference between thejunction of the flow restricting devices and the reaction chamber tomaintain a constant pressure at the junction and thus regulates in anindirect but highly accurate manner the flow rate of sample mixture intothe reaction chamber. The sample mixture reaching the reaction chamberpasses only through the flow restricting devices, both of which havefixed restrictions and can be made of inert materials such as glass.

Proper flow control over a wide range of sample mixture source pressuresand improved life are achieved by including the regulating device in atube connecting the inlet of the bypass pump to the atmosphere. Theinlet pressure of the bypass pump is controlled by regulating air flowthrough the tube and that inlet pressure in turn determines the pressureat the junction of the flow restricting devices. None of the samplemixture contacts the regulating device.

A flow indicating or rectifying device preferably is located between thejunction of the flow restricting devices and the inlet of the bypasspump to provide a visual indication that adequate sample mixture isflowing to the reaction chamber. Such devices are highly desirable sincethe regulating device usually is capable of regulating the pressure atthe junction to a value indicative of proper flow even though no samplemixture is reaching the junction.

The flow control system is useful with any analytical instrument thatrequires constant gas flow rates and the term. reaction as used hereinis intended to include flame ionization processes, light absorption andscattering processes, chemiluminescent processes, etc. An importantapplication of the system lies in the use of the aforementionedchemiluminescent detector for analyzing exhaust gases from vehicles. Acertain volume of the exhaust gases usually is collected and at somelater time that volume is analyzed for various gaseous constituentsincluding nitric oxide and nitrogen dioxide. If rigid collectioncontainers are used, depletion of the volume during analysis-reduces thepressure of the remainder and thereby tends to reduce flow rates intothe detector. Analyzing exhaust gases directly from the vehicle exhaustsystem also produces pressure changes that are eliminated by the systemof this invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustrating theuse of the flow control system of this invention in a chemiluminescentdetector. FIG. 2 is a detail of an alternate bypass flow mechanism inwhich the regulating device is located in a tube connecting the bypasspump inlet with the atmosphere. The regulating device uses reactionchamber pressure as the reference pressure so that the device in effectresponds to the pressure drop across the downstream flow restriction.FIG. 3 is a detail of a bypass system in which the regulating device islocated in the line connecting the bypass pump to the junction of theflow restricting devices. Some sample mixture contacts the regulator ofFIG. 3, but that sample mixture does not participate in the analyticalreaction.

DETAILED DESCRIPTION Referring to FIGS. 1 and 2, a chemiluminescentdetector for measuring amounts of nitric oxide in a gaseous samplemixture by reaction with an oxygen-ozone reactant mixture comprises areaction chamber 10. A sample conduit 12 and a reactant conduit 14connect with the interior of reaction chamber 10. A light transmittingelement 16 is located'in one wall of reaction chamber 10 and a lightdetecting device 18 is located outside of light transmitting element 16.An electrical power supply 20 and amplifying and readout circuitry 22are connected to light detecting device 18.

Reactant conduit 14 is connected through a capillary 26 to an ozonegenerator 28. Ozone generator 28 is supplied with electrical power by apower source 30. A regulating device 32 connects ozone generator 28 withan oxygen source 34 and a presure gauge 36 indicates the pressurebetween regulating device 32 and ozone generator 28.

-Sample conduit 12 is connected to a first or downstream capillary 40.Capillary 40 is in series with an up stream capillary 42 and capillary42 is connected through a filter 44 to the gaseous sample mixture source46. A pressure gauge 47 is connected to the junction of capillaries 40and 42. A pressure gauge 48 is connected to the outlet 50 of thereaction chamber and an exhaust pump 52 connects outlet 50 with theatmosphere.

A bypass flow mechanism indicated by dashed line 54 comprises arotameter 56 connected to the junction of capillary 40 with capillary42. The outlet of rotameter 56 is connected to the inlet 58 of a bypasspump 60. Inlet 58 is connected through a pressure regulating device 62and an air filter 64 to the atmosphere. Pressure regulating device 62 isconnected to outlet 50 of the reaction chamber 10 and receives therefroma reference pressure substantially equal to the pressure within reactionchamber 10.

As shown in FIG. 2, regulating device 62 comprises a housing 68 having amovable diaphragm 70 located therein to divide the interior into areference chamber 72 and a controlled pressure chamber 74. A baffle 76having a valve seat 78 separates the chamber 74 from an inlet airchamber 80. Reference chamber 72 is connected to reaction chamber 10,chamber 74 is connected to the inlet of pump 60, and chamber 80 isconnected to the atmosphere through filter 64. A valve member 82 isconnected to diaphragm 70 and a compressive spring 84 located inreference chamber 72 urges valve member 82 away from valve seat 78.

Pumps 52 and 60 both preferably are of the constant volume type.Capillaries 26 and 40 both provide relatively high flow restrictionswhile capillary 42 provides a somewhat lower flow restriction. Each ofthe capillaries typically is made of glass. Filter 44 preferably is madeof porous sintered inert metal; stainless steel operated atapproximately room temperature is suitable. Substantially pure oxygen issupplied by oxygen source 34 and ozone generator 28 produces a reactantmixture containing about 2 percent ozone with the balance oxygen.

Operation of the detector occurs in the following manner. Pumps 52 and60 are started and ozone generator 28 is activated. Regulator 32 isadjusted to achieve a predetermined reading on gauge 36 that is known toproduce the desired flow rate of reactant mixture into the reactionchamber at the operating pressure of the reaction chamber. A typicalreaction chamber pressure is about Torr and a typical reactant mixtureflow rate is about 120 cc. per min.

Gaseous sample mixture is drawn from source 46 through filter 44 andcapillary 42 to the junction of capillary 42 with capillary 40. Pump 52draws some of the sample mixture reaching the junction through capillary40 and into the reaction chamber. Pump 60 draws off sample mixture fromthe junction and draws air through filter 64 and regulator 62. Regulator62 is adjusted manually to obtain a predetermined pressure reading ongauge 47, typically about 600-700 mm Hg. absolute, that produces thedesired flow rate of sample mixture into the reaction chamber.

Regulating device 62 then begins monitoring the pressure differencebetween the junction of the capillaries and the reference pressure inthe reaction chamber. Any decline in the pressure at the junction of thecapillaries is converted by regulating device 62 into an increased airflow through the regulating device. Increased air flow through theregulating device reduces the amount of sample mixture flow from thejunction of the capillaries to the inlet of the bypass pump and therebyrestores the desired pressure at the junction of the capillaries. Theconstant pressure at the junction of the capillaries insures constantflow through capillary 40. Similar operation compensates for anyincreases in the pressure at the junction of the capillaries.

I-Ialting sample mixture flow between sample source 46 and the junctionof the capillaries is manifested by a reduction in the pressure at thejunction. In the absence of rotameter 56, regulating device 62 cancompensate for the pressure reduction in the manner described abovewithout providing any external indication of the fact that no samplemixture is flowing into the reaction chamber. Including the rotameterand obtaining a flow indication therefrom avoids such errors.

In FIG. 3, a regulating device 62a is constructed similarly toregulating device 62 except that baffle 76a separates a controlledpressure chamber 74a from a pump inlet chamber 86 and spring 84a urgesvalve member 820 onto its valve seat 78a. Chamber 74a is connected tothe outlet of rotameter 56 and chamber 86 is connected to the pumpinlet. Operation occurs in a manner similar to that described aboveexcept that regulation is achieved by varying sample mixture flowthrough the regulating device and the bypass pump instead of varying airflow through an auxiliary line. The life of the regulating device ofFIG. 3 is less than that of FIG. 2 but the FIG. 3 system avoids anycontamination of the sample mixture reaching the reaction chamber by theflexible diaphragm of the regulating device.

Other flow indicating devices can be substituted for rotameter 56 ifdesired. Note also that the ability of a good rotameter to function as acheck valve to prevent flow into the junctions of the capillaries alsocan be used to determine if proper sample mixture is reaching thejunction of the capillaries. In such a system, an undue restriction incapillary 42 or upstream thereof is manifested by a declining pressureindication on gauge 47 which alerts the operator to the fact thatinsufficient sample mixture is reaching the reaction chamber (see FIG.3). A check valve can be substituted for such a rotameter if desired.Controlling the pressure regulating device across the downstreamcapillary as shown permits the regulating device to maintain thepressure at the junction of the capillaries as a function of thepressure drop through the downstream capillary without any interferencefrom the atmosphere or other system components.

Thus this invention provides a gas flow control system for an analyticalinstrument that maintains automatically a constant flow rate of samplemixture into the reaction chamber. The invention is useful particularlyin a chemiluminescent detector but it also can be used in otheranalytical instruments requiring a constant sample mixture flow rate.

I claim:

1. in an instrument for measuring analytically the amount of a gaseousconstituent in a gaseous sample mixture by introducing the gaseoussample mixture into a reaction chamber and measuring a characteristic ofthe gaseous constituent within the reaction chamber, a flow controlsystem for maintaining accurate flow of the gaseous sample mixture intothe reaction chamber comprising an upstream flow restricting device anda downstream flow restricting device in series with each other forconducting the gaseous sample mixture to the reaction chamber,

bypass flow means connected to the junction of the upstream flowrestricting device with the downstream flow restricting device, saidbypass flow means including a bypass pump for drawing off a portion ofthe gaseous sample mixture that has passed through the upstream flowrestricting device and regulating means responsive to the pressuredifference between the junction of the flow restricting devices and thereaction chamber for regulating the pressure at the junction of the flowrestricting devices, and

an exhaust pump connected to the reaction chamber for drawing gaseoussample mixture from the junction of the flow restricting devices throughthe downstream flow restricting device and into'the reaction chamber.

2. The instrument of claim 1 in which the bypass flow means comprises aflow indicating means located be tween the junction of the flowrestricting devices and the inlet of the bypass pump.

3. The instrument of claim 1 in which each of the flow restrictingdevices is a glass capillary tube.

4. The instrument of claim 1 in which the reaction occurring within thereaction chamber produces chemiluminescence.

1. In an instrument for measuring analytically the amount of a gaseousconstituent in a gaseous sample mixture by introducing the gaseoussample mixture into a reaction chamber and measuring a characteristic ofthe gaseous constituent within the reaction chamber, a flow controlsystem for maintaining accurate flow of the gaseous sample mixture intothe reaction chamber comprising an upstream flow restricting device anda downstream flow restricting device in series with each other forconducting the gaseous sample mixture to the reaction chamber, bypassflow means connected to the junction of the upstream flow restrictingdevice with the downstream flow restricting device, said bypass flowmeans including a bypass pump for drawing off a portion of the gaseoussample mixture that has passed through the upstream flow restrictingdevice and regulating means responsive to the pressure differencebetween the junction of the flow rEstricting devices and the reactionchamber for regulating the pressure at the junction of the flowrestricting devices, and an exhaust pump connected to the reactionchamber for drawing gaseous sample mixture from the junction of the flowrestricting devices through the downstream flow restricting device andinto the reaction chamber.
 2. The instrument of claim 1 in which thebypass flow means comprises a flow indicating means located between thejunction of the flow restricting devices and the inlet of the bypasspump.
 3. The instrument of claim 1 in which each of the flow restrictingdevices is a glass capillary tube.
 4. The instrument of claim 1 in whichthe reaction occurring within the reaction chamber produceschemiluminescence.